1. Technical Field
The present invention relates generally to gas turbine engines having centrifugal compressors and, more specifically, supplying turbine cooling air from a centrifugal compressor.
2. Background Information
A conventional gas turbine engine typically includes a compressor, combustor and turbine, both rotating turbine components such as blades, disks and retainers, and stationary turbine components such as vanes, shrouds and frames routinely require cooling due to heating thereof by hot combustion gases. Cooling of the turbine, especially the rotating components, is important to the proper function and safe operation of the engine. Failure to adequately cool a turbine disk and its blading, for example, by providing cooling air deficient in supply pressure, volumetric flow rate or temperature margin, may be detrimental to the life and mechanical integrity of the turbine. Depending on the nature and extent of the cooling deficiency, the impact on engine operation may range from relatively benign blade tip distress, resulting in a reduction in engine power and useable blade life, to a rupture of a turbine disk, resulting in an unscheduled engine shutdown.
Balanced with the need to adequately cool the turbine is the desire for higher levels of engine operating efficiency which translate into lower fuel consumption and lower operating costs. Since turbine cooling air is typically drawn from one or more stages of the compressor and channelled by various means such as pipes, ducts and internal passageways to the desired components, such air is not available to be mixed with fuel, ignited in the combustor and undergo work extraction in the primary gas flowpath of the turbine. Total cooling flow bled from the compressor is a loss in the engine operating cycle, and it is desirable to keep such losses to a minimum.
Some conventional engines employ clean air bleed systems to cool turbine components in gas turbines using an axi-centrifugal compressor as is done in the General Electric CFE738 engine. The turbine cooling supply air exits the centrifugal diffuser through a small gap between the diffuser exit and deswirler inner shroud. This air is then ducted radially inward by expensive integrally cast passages to the inside of the inner combustion case where it is then ducted into an accelerator via an arduous path where the airflow must make several 90 degree turns generating losses (and thus raising the temperature of the cooling air) before going through the accelerator. After leaving the accelerator, this cooling air travels up along a first stage turbine disk into a first stage turbine blade. The various turns of the cooling air are a loss in the engine operating cycle, and it is desirable to keep such losses to a minimum.